The present invention relates to an oil-country-tubular threaded joint, particularly, to an oil-country-tubular threaded joint having highly improved characteristics in regard to the prevention of fluid leakage even in an environment where various forces such as external pressure, tensile forces, compression forces, and bending stresses are exerted thereon. The present invention also relates to a thread-machining method for the oil-country-tubular threaded joint and to a thread-machining apparatus therefor; particularly, the invention relates to a technique of machining and quickly completing female screw threads in the oil-country-tubular threaded joint by using a tool-rotating thread-machining apparatus.
Presently, threaded joints are widely used as in techniques for coupling oil-country-tubulars used for research and production relating to natural resources, such as natural gas and crude oil, at depth of thousands of meters. Oil-country-tubulars (which may be simply called xe2x80x9ctubexe2x80x9d) are used in severe environments where high pressures and high loads are exerted. Under these circumstances, threaded joints for the tube are required to have characteristics, for example, to withstand forces such as tensile forces due to the weight of coupled tube in the axial direction, and external pressures, and to be repeatedly usable for dozens of times.
Generally, there are two basic types of oil-country-tubular threaded joints. One is an integral type in which a pin section having male screw threads and a box section having female screw threads are arranged, respectively, on ends of tube, with which the tube are coupled. Another is a coupling type in which, as shown in FIG. 1, a coupling 2 (having male screw threads) having a box section 1 on each end is used to couple tube 4 having a pin section (having male screw threads) at the end.
For the shape of threads in the threaded joints, trapezoidal screw threads according to the API (American Petroleum Institute) standards are generally used. For example, as shown in FIGS. 2A to 2C, when a male screw thread 5 and a female screw thread 6 are tightened and engaged with each other, a load face 7 is formed of respective load faces 7b and 7a of the male screw thread 5 and the female screw thread 6. Similarly, an insertion face 8 is formed of insertion face 8b and 8a of the male screw thread 5 and the female screw thread 6, and a crest face 9 of a thread is formed of a root face 10d of the female screw thread 6 and a crest face 9b of the male screw thread 5.
In this case, an angle formed by the load face 7 or the insertion face 8 with a line perpendicular to the axis of the casing is referred to a flank angle (the angle on the load face 7 is referred to as a load flank angle xcex1; the angle on the thread insertion face 8 is referred to as a stubbing angle xcex2). These angles are represented by positive or negative numbers (values) For the load flank angle xcex1, counterclockwise angles are represented by negative numbers (values); for the stubbing flank angle, counterclockwise angles are represented by positive numbers (values).
The trapezoidal screw threads according to the API standards are formed with a load flank angle xcex1 of 3xc2x0 and a stubbing flank angle xcex2 of 10xc2x0. As shown in FIG. 2(c), when the trapezoidal screw threads are tightened, the thread faces contact the load face 7, the thread faces do not contact creating a clearance, and the thread faces contact on at least one of the crest face 9 and the root face 10.
In recent constructions of the oil-country-tubulars 4, compression forces are exerted frequently in the axial direction of the oil-country-tubulars. In this condition, when tensile forces or bending forces impinge on the tube, the API trapezoidal screw threads are apt to be disengaged, possibly causing a problem in that fluid (such as natural gas or crude oil) containing therein leaks outside.
To overcome the above problem, solutions have been proposed. For example, according to Japanese Unexamined Patent Publications Nos. 6-281059 and 6-281061, the load flank angle xcex1 of a load face 7 is arranged to be negative for preventing the disengagement of threads. Such screw threads with a negative flank angle are called negative screw threads or hook screw threads.
When a negative load flank angle xcex1 is provided, however, the load faces 7b and 7a of the male screw thread 5 and the female screw thread 6 contact firmly, causing stress concentration. This causes damage to corner sections of the treads, which is a cause of leakage of gas or liquid. Under these circumstances, there are increasing demands for threaded joints that can also withstand compression and bending forces.
To practically use the negative screw threads, however, many problems still remain. One of the problems relates to the thread-machining technique. Currently, for machining negative screw threads, similarly to the case of a lathe, only a single machining tool (generally called a xe2x80x9cchaserxe2x80x9d) is fitted to a machine. In this case, a tubular body, which is a thread-machining object (which may be called a xe2x80x9cworkxe2x80x9d, depending on the case) is rotated, and at the same time, is reciprocated in contact with the chaser. Also, a chaser 11 has a single machining edge, or at most two cutting edges. With this type of chaser, as many as 6 to 12 strokes (expressed also as 6 to 12 passes) of reciprocating operations must be repeated to complete thread-machining. That is, as shown in FIG. 3, since a thread 14 has a negative flank angle 15, each pass requires adjustment of an entering volume A in the radial direction and a movement volume (b) in the axial direction by the negative angle, and also, a machining time longer than for normal cases is required. Furthermore, when the joints produced as described above are used, deformations of a flank face 16 are apt to occur, and the joints are therefore moved by forces, particularly, compression forces and tensile forces, thereby producing a problem in that the joints frequently leak fluid.
In view of the above problems, the present invention provides an oil-country-tubular threaded joint that can withstand tensile forces, and in addition, other forces such as compression and bending forces, thus producing highly improved characteristics in regard to the prevention of fluid leakage. With the oil-country-tubular threaded joint, no damage will be caused to corners of threads while the oil-country-tubulars are being fabricated or are being used.
Also, the present invention provides a thread-machining method and a thread-machining apparatus for the oil-country-tubular threaded joint which allow negative screw threads of a higher quality level than that of conventional cases in reduced time.
To these ends, according to one aspect of the present invention, there is provided an oil-country-tubular threaded joint having male screw threads and female screw threads which have a load face formed of the male screw thread and the female screw thread, which has load flank angles of negative values, in which the load flank angles are varied at a position at which the load face is divided in the height direction of the male screw thread into an upper side and a lower side so that an upper load flank angle is smaller than a lower load flank angle.
Also, there is provided an oil-country-tubular threaded joint having male screw threads and female screw threads which have a load face formed of the male screw thread and the female screw thread, which has load flank angles of negative values, in which the load flank angles of the male screw thread and the female screw thread are different from each other.
Also, according to another aspect of the present invention, there is provided a thread-machining method for the oil-country-tubular, which rotates the rotational body to perform thread-machining by using a plurality of chasers, in which differently shaped cutting edges are arranged on the chasers sequentially, and the chasers are fitted to the rotational body and are used from the beginning to the end of thread-machining in forming screw threads of oil-country-tubulars.
Also, according to still another aspect of the present invention, there is provided a thread-machining apparatus for the oil-country-tubular threaded joint, in which a rotational body has a function to rotate on the central axis, a function to move in the axial direction of the central axis, and a plurality of chasers on which differently shaped cutting edges are arranged sequentially.